Integrated optical waveguide circuits, analogous to integrated electronic circuits, comprise optical waveguides formed on a substrate. Modulation of light propagating in these waveguides is achieved by actively altering the optical properties of the waveguide circuit media.
The application of integrated optics is most common in fibre optic communication, though many other applications exist. Common optical functions for which integrated optics is utilised include directional switching, phase modulation and intensity modulation.
Several active integrated optical systems have been based on silicon. The advantages of silicon integrated optical devices include the potential use of standard silicon integrated electronic circuit manufacturing technology and the integration of optical and electronic circuits on one silicon device. For the effective use of silicon integrated optics it is considered important to produce a low loss waveguide structure (i.e. less than 1 dB/cm) with electrically controllable modulation utilising standard planar silicon electronic integrated circuit manufacturing technology. The prior art has so far failed to fully satisfy these combined requirements.
U.S. Pat. No. 4,746,183 and U.S. Pat. No. 4,787,691 describe a number of active waveguide devices utilising a vertical doped junction in a silicon rib waveguide, i.e. a diode formed between an electrode on the upper surface of the rib and an electrode on the opposite side of the device. The devices may be constructed using a highly doped substrate, acting as a lower waveguide cladding but these will suffer from high optical losses due to the high free carrier absorption of the guided wave's evanescent field travelling in the substrate. An alternative structure uses a buried silicon dioxide cladding which has lower optical losses (providing the buried silicon dioxide layer is thick enough to fully confine the guided wave). However, this arrangement requires additional manufacturing steps in order to make a break in the buried insulator layer to provide an electrical contact to a low resistance substrate.
EP-A-0,433,552 describes an active silicon waveguide device constructed in a rib waveguide on silicon dioxide. In a first arrangement a vertical p/n junction is formed in the rib with one electrical contact on the upper surface of the rib and another (in the form of a heavily doped n-region) formed on the upper surface of the silicon layer adjacent the rib. The current flow between these electrical contacts (which alters the charge carrier density in the waveguide) does not therefore extend across the whole of the cross-sectional area of the rib. This may be of little importance for devices of sub-micron dimensions (as described in this prior art) but with devices which are large enough to be compatible with fibre optics (which typically have a core section diameter of around 8 microns), this reduces the overlap between the charge carriers and the guided wave which reduces the effective refractive index change of the waveguide for a given current. This may mean that the device has to be operated in a current saturation mode, which reduces switching speed, in order to achieve a useful refractive index change in the waveguide.
In another arrangement described in EP-A-0433552, a lateral bipolar transistor is provided in a planar waveguide comprising a silicon layer over a layer of silicon dioxide on a silicon substrate. This arrangement leads to a waveguide structure with high optical losses due to the absorption of the guided waves evanescent field in the highly doped regions of the transistor. Also, such a structure can only be formed in a planar waveguide less than one micron thick as it is difficult to introduce dopants in evenly distributed concentrations for a depth greater than around 0.5 microns.
Silicon waveguide modulation devices which utilise field effect transistors to vary free carrier concentration in the waveguides by free carrier injection and depletion have also been proposed (as in GB-A-2,230,616 for instance). However, these devices require waveguides with sub-micron dimensions so precluding these devices from applications requiring low-loss connections with optical fibres and means that the devices are expensive to manufacture as they cannot be formed from readily available silicon on silicon dioxide wafers for reasons which will be explained in more detail.